The present invention relates to a system for detecting the position of a read-write head in a disk memory apparatus such as a magnetic disk apparatus or an optical disk apparatus.
Magnetic and optical disks have been widely used as memory devices. A large number of concentric data tracks are formed on the disk. In order to access a selected track, a read-write head must be quickly and accurately placed on the selected track. A tracking servo system is used for this purpose.
Various types of data surface servo systems have been developed wherein servo data is recorded on the data surface. A typical example is an embedded servo system wherein a disk data surface is divided into a plurality of sectors (e.g., 32 sectors), and servo data is recorded on servo sectors each embedded between two adjacent data sectors. According to the embedded servo system, the disk and servo data processing circuit may be simple in construction compared with any other data surface servo systems.
With the embedded servo system, since the servo data is accessed only from the servo sectors, continuous positioning control of the head cannot be performed. For this reason, when the head is moved at high speed, position signals based on the servo data are obtained incompletely. As a result, the head speed control and head positioning with respect to a target track will not be performed. Therefore, the head speed in the radial direction of the disk is limited.
In my copending U.S. patent application No. 469,144, filed Feb. 23, 1983, now U.S. Pat. No. 4,499,511, dated Feb. 12, 1985 assigned to the same assignee as this application and entitled "SYSTEM FOR DETECTING POSITION OF A READ-WRITE HEAD IN SEEK OPERATION ON A DISK MEMORY HAVING DATA AND SERVO SECTORS", embedded servo sectors are disclosed which have such servo information patterns formed thereon as to enable the head to accurately seek a target track at a speed of less than 4 tracks/sector, 8 tracks/sector or 16 tracks/sector. In order to shorten the seek time, the speed of the head must be increased. For this purpose, the number of servo data patterns formed in the servo sector must be increased. An increase in the number of servo data patterns, formed in the servo sector, results in an increase in the servo sector region. In other words, the data sector region is decreased so that the recording density of data is lowered. An additional disadvantage is that a complex servo control circuit is required.
In the servo sector (FIG. 13) of the above-mentioned copending application, which is adapted for the head speed of less than 16 tracks/sector, three types of servo patterns are formed which have periods of four, eight and sixteen servo tracks, respectively. Each of the periodic servo patterns has a pair of alternate pattern arrays having the same period and arranged in a staggered fashion. The basic servo pattern having a period of four tracks is formed in a two-phase manner, so that eight servo pattern arrays are arranged. According to the teaching of this example, in order to adapt for the head speed of 32 tracks/sector, a pair of alternate arrays having a period of 32 tracks is added in the servo sector, so that 10 servo pattern arrays are required. A detecting means is provided for each array of the servo patterns, thus complicating the servo circuit.